Here’s the thing about Steffon Baratheon…after WOIAF, I’m less convinced than I was before that Steffon was part of the Southron Ambitions conspiracy. After all, everything in WOIAF speaks to Steffon as having been super-tight with another power bloc:

Aerys and Tywin had known each other since childhood. As a boy, Tywin Lannister had served as a royal page at King’s Landing. He and Prince Aerys, together with a younger page, the prince’s cousin Steffon Baratheon of Storm’s End, had become inseparable. During the War of the Ninepenny Kings, the three friends had fought together, Tywin as a new-made knight, Steffon and Prince Aerys as squires.

By contrast, we learn absolutely nothing about any potential links to Jon Arryn, Hoster Tully, or Rickard Stark. Now, it’s possible that Steffon’s closeness to Aerys meant that he could see the growing madness, but that’s not really the impression you get with the whole trip to Volantis. (Maybe he blamed the Targs for his father’s death? I dunno.)The detail that bugs me is we don’t know why Jon Arryn was chosen to be Robert’s foster-father – was this due to a close relationship between Jon and Steffon or something else?

So my current thinking – and I have to say, I feel 60-40 on this – the Baratheons weren’t supposed to be a part of the SA coalition: after all, they had close blood relations to the Targs, Steffon was personally close to Aerys, etc. Rather in a stroke of good luck/political nous, it was only after Steffon’s death and Robert becoming Jon Arryn’s ward that the SAs could count on having the Stormlands in their camp. 

Perhaps this explains why Hoster Tully reached out to Tywin Lannister for a marriage alliance. The SAs not being idiots, they could see that Tywin was on the outs with Aerys, and getting the second-most powerful man in Westerosi politics along with the might of the Westerlands for their power bloc could only help.